I have read that the steel ones are consistently closer to the stated bores, not that it matters since you just use the one that fits rather that breaking out the micrometer on your paint and sizer when you play.
Yes
Stainless inserts are machined to the size
Aluminium inserts are anodised so must either be machined a fraction larger to accommodate the finish, or if machined to the size will be a fraction smaller and tighter by the depth of the anodised finish
In reality we are using sizing kits to our preferred match/over/under bore for a gelatine ball that clearly isn't really 0.68" otherwise we wouldn't bother with sizing kits in the first place
Freaks and other sizing kits allow us to adjust as we wish on the day, and possibly throughout the day